Digital printing has revolutionized the printing industry. The ease of printing variable images, making reprints, archiving images, and printing on demand are some of the key advantages of digital printing.
Ink jet printing is one of the cheapest and most convenient technologies available for digital printing. Ink jet printers form an image by delivering small droplets of liquid ink through an ink delivery head. The ink generally contains either soluble dyes or insoluble pigments as colorants, and a solvent. Many commonly used inks contain water as a component. Other inks contain volatile organic solvents. Still other inks contain UV curable monomers.
The speed of solvent removal from a printed surface can affect the quality of the resultant image. Slow-drying ink can lead to coalescence of printed ink droplets, which may negatively affect print quality. The speed of solvent removal is affected by the amount and type of solvent in the ink and the absorptiveness of the printed surface. Generally, an absorptive surface enhances solvent removal.
Generally, inks that contain volatile organic solvents dry more quickly than water-based inks. However, inks containing volatile organic solvents may pose health and safety hazards. Therefore, such inks are generally not suitable for use in an office environment. Instead, organic solvent or monomer-based inks are typically used in an industrial environment using proper handling and safety measures. Water-based inks are preferred for use in office environments.
Plastic cards are increasingly being used as data carrying devices, for example, for identification and electronic transactions. Common examples of such data carrying devices are credit cards, ATM cards, ID cards, badges, membership cards, access cards etc. . . . Advanced electronic technologies are making these cards increasingly valuable and sophisticated. Besides incorporating data in the cards, the cards are used as billboards to advertise the business of the issuer. Additionally, the cards are frequently personalized to include unique information about the card user. It is desirable to produce such cards with high quality print and high durability.
A majority of card personalization and issuance is performed in an office environment. Typically, a non-porous plastic card surface is personalized by thermal transfer printing.
Ink jet printing provides a flexible and economically attractive option for card printing. Attempts have been made to print an image on the non-porous plastic surface of a card using a water-based ink and a coating that provides an absorptive layer. Although an acceptable print quality can be achieved using this method, the absorptive layer tends to continue absorbing moisture over the life of the card and may adversely affect card durability. When dye-based inks are used, the absorbed moisture can cause the dyes to migrate, thereby adversely affecting image quality. Furthermore, the absorptive layer tends to become increasingly soft as more moisture is absorbed such that it can easily be scraped or scratched during use. In most cases, application of a protective layer, such as a coating or overlaminate, still does not provide adequate protection in hot and humid environments.
The invention provides a method for printing an image on a substrate. The method is useful for printing a variety of substrates, in particular, non-porous substrates, such as plastics, for example data-carrying devices.
According to the invention, a carrier substrate is coated with a receptive layer. The receptive layer preferably includes at least two layers: a first transferable skin layer and a second absorptive layer. When applied to the carrier substrate, the receptive layer is positioned such that the absorptive later is located between the transferable skin layer and the carrier substrate. If desired, the receptive layer can include an intermediate layer, located between the transferable skin layer and the absorptive layer.
According to the invention, an image is printed on the transferable skin layer. Typically, liquid inks, such as ink jet ink, are used. The transferable skin layer allows the solvent to pass through to the absorptive layer, while collecting the colorant. Thus, the absorptive layer helps the printed image to dry while the colorant is retained by the transferable skin layer. Additionally, the drying process may be further enhanced with the assistance of an external heat source, circulating air (heated or unheated), radiation, etc.
Once the image is substantially dry, such that it will not smear or smudge during handling, the image is transferred to a final substrate. To transfer the image, at least some, substantially all, or all of the transferable skin layer (on which the image is printed) is transferred to a final substrate. All or most of the absorptive layer and absorbed solvents remain on the carrier substrate. If present, the intermediate layer may or may not, in whole or in part, transfer to the final substrate during print transfer.
Thus, a durable image is formed on the final substrate with a substantially thinner water-absorbing layer than other available water-based, ink jet printed devices. The durability of the print on the final substrate can be further improved by application of a protective layer such as a topcoat or overlaminate.